The present invention relates to a permanent-magnet rotor for electric motors, and to a method of making a permanent-magnet rotor.
As is known, electric motors with permanent-magnet rotors are currently used in the field of small centrifugal pumps.
The magnets that are used are produced by sintering appropriate powders under the action of a magnetic field.
For motors having rotors with an axial size of up to approximately 3 cm, the magnets are manufactured with a tubular shape by filling an appropriate mold with the powders and by applying axial compression.
The compression is such that the axial dimension of the finished part is approximately one third of the dimension occupied by the powders when mold filling ends.
For electric motors with a rotor having greater axial dimensions it becomes practically impossible to obtain the rotor with the above described method in an economically and qualitatively satisfactory manner, and therefore it is preferred to manufacture semitubular parts to be assembled so as to compose the tubular rotor.
These semitubular parts are again obtained by sintering powders in a magnetic field, but in this case compression occurs at right angles to the axial direction.
For rotor assembly, initially the two semitubular permanent magnets used to be enclosed in a tubular casing made of stainless steel which, being a diamagnetic material, in theory has no effect on the magnetic field produced by the stator of the motor.
However, practical tests have shown that a dielectric forms in the air gap between the magnets and the pole pieces due to the presence of the stainless steel casing and of the wall of the rotor chamber, causing a 30% drop in efficiency.
An attempt was later made to embed the two semitubular permanent magnets in an injection-molded plastic casing, but even this attempt failed to yield the expected results due to the fact that the necessary minimum thicknesses of the plastic material and the significant axial size of the rotor, combined with the rather low fluidity of the injection-molded material, prevent the complete filling of the intended regions and thus give an unacceptable final result.
Increase in injection pressures to achieve complete filling caused cracking of the magnets due to their great hardness and consequent great fragility.